1. Field of the Invention
This invention relates to electrical contacting surfaces for low voltage and/or high current applications that may be connected and disconnected multiple times. The electrical contacting surfaces of the present invention employ numerous electrically conductive substantially spherical protrusions and may be used to establish multiple parallel electrically conductive pathways to other electrically conductive contacting surfaces.
2. Description of the Related Art
Electrical conductivity is a property common to many materials including metals. An electrically conductive material is a substance that allows the flow of electrical charge throughout its mass. Electrical charges come in many forms and may result from the separation of electrons from atoms. The separation of charge from atoms can render a substance electrically conductive if the charges are free to move. Although charged atoms can conduct electricity, in many cases it is the flow of electrons rather than charged atoms throughout a substance that is responsible for electrical conductivity.
Many of the elements in the periodic table are metals. Metallic elements such as copper are good conductors of electricity because they support the flow of electrons throughout their mass. This may result from loose electrons that are free to travel between atoms. Generally speaking all of the true metallic elements in the periodic table are capable of conducting electricity in this manner. Some elements such as silver and copper are good conductors of electricity while others such as lead tend to be significantly less conductive. It should be noted that the morphology of the metal itself may play a role in conductivity.
Generally speaking metals conduct electricity throughout their entire mass. When two or more pieces of metal are placed together good electrical contact between them may or may not occur. This depends on several factors including contact surface area, number of contacting points, surface contamination, contact pressure, and applied voltage.
Sliding electrical contactors are electrical contactors that have electrically conductive sliding surfaces that slide together. In many instances pressure is provided between the two contacting surfaces in order to improve conductivity. Examples of this include the following:                1. Wall outlets and matching plugs for providing power to household appliances.        2. Sliding connectors for connecting one set of wires to another.        3. Sliding contacts used to provide electrical connections to printed circuit boards.        4. Electric switches including knife switches and the like.        
Wall outlets are sliding electrical contactors that are used to provide electric power to household electrical devices such as appliances. The outlets themselves are usually mounted flush to the surfaces of walls comprising the interior spaces of buildings. The flush mounting characteristics provide good aesthetic properties as well as significantly reducing the likelihood of damage resulting from inadvertently bumping into them. Many electrical outlets have two complete sets of electrical contactors. Each set of contactors having two relatively narrow slots with inner metal contacting surfaces along with a third and somewhat more circular hole having inner metal contacting surfaces as well. The inner metal contacting surfaces of the two relatively narrow slots are used to provide electric power to matching metal prongs found on the plugs of household electrical devices such as appliances. The inner metal contacting surfaces of the third somewhat more circular hole are used to provide an electric ground connection to the matching prong found on certain plugs that may be used to provide ground connections to household electrical devices such as appliances.
The two basic types of electric power are AC power and DC power. AC power (alternating current) continuously changes voltage with the voltage reversing itself at regular intervals over time. DC power (direct current) is steady with the voltage remaining constant over time.
The electric power provided to most standard household wall outlets is about 115 volts AC. This voltage represents the root mean squared voltage of a sixty cycle per second AC waveform. The peak voltage is usually about 170 volts (considerably higher than the root mean squared voltage). Because of this, household electrical outlets and any electrical devices that use this power require suitable electrical insulation properties that can safely withstand voltages in excess of 170.
Electrical grounding is provided because contact with 115 volts AC may result in serious bodily injury or even death. Electric shock occurs when a potential voltage exists across the body that is sufficient to carry a current disruptive to normal bodily functions. The nervous system of the human body is controlled by electrical impulses that may be considerably less than one milliampere (0.001 amperes). A potential of 115 volts AC is sufficient to carry several milliamperes across body parts under most conditions and may carry significantly greater currents through wet contacting surfaces. Once a disruptive current is established across the body, the individual may not be able to let go. In this case the current resulting from electric shock may be sufficient to completely overwhelm the nervous system.
Although an electric voltage potential must exist across the body in order to carry electric current, contact with only one electrically charged surface may result in electric shock. One reason for this has to do with electrical ground. An electrical ground such as the surface of the earth and conductive materials in contact with the surface of the earth represents an electrical connection that a voltage potential may be established across. If a metal electric appliance has an internal connection to its voltage source (from a bad wire or component) the entire outer conductive metal surface may acquire sufficient voltage with respect to ground to deliver an electric shock. This electric shock hazard may be reduced by connecting the metal conductive outer surface of the electric appliance to ground. A proper ground connection can prevent this hazardous condition thereby reducing the likelihood of electric shock.
Certain situations may arise in which a grounded electrical device may not be properly grounded. This condition can occur if the connection between the ground and the device is faulty. The sources of possible bad connections that lead up to this condition are numerous including a bad earth ground connection, faulty wiring, a bad connection between the outlet and plug, a faulty electrical cord, and a bad connection between the internal grounding wire of the electrical device and the device housing. Good electrical contact between the inner grounding wire and the housings of an electrical device may be achieved by placing a washer having numerous sharp edged cutting surfaces between the housing and internal grounding wire and tightening with a nut and bolt. The grounding washer is designed to cut into metal surfaces to establish good continuity. It is interesting to note that significant effort has been placed on providing a good ground connection between internal grounding wires and metal housings but not much effort has been placed on the grounding connection between the grounding prongs of plugs and the sliding electric contacting surfaces on the interior surfaces of outlets. One reason for this may have to do with the fact that the internal ground connection of metal housings is meant to be permanent and the plug connection is designed to be removable.
The 115 volts AC in use today results from several needs including the following:                1. The ability to change voltage by employing transformers.        2. The need to carry considerable power through relatively small diameter wires.        3. The need to provide electric lighting that does not flicker.        4. The need to keep voltages down to a “reasonable level”        5. The need to reduce arcing in switch contacts.        6. Sufficient voltage to overcome surface contamination between electrical contacting surfaces.        
Transformers are devices that change AC voltages. A transformer consists of insulated copper wire wrapped around an iron core. When a voltage is applied across the wire, a magnetic field is established in accordance with the right hand rule of electrically induced magnetism. This magnetic field will rise and fall and reverse direction with each cycle of the AC waveform. This changing magnetic field may be conducted to a second copper insulated wire wrapped around the same iron core. A changing AC voltage is established in the second copper wire wrapping (coil) as a result of the changing magnetic field present in the iron core. The voltage ratio of the transformer is based on the ratio of turns between the two separate coils. In order to induce a current in an electrical conductor (such as a coil of wire) the magnetic field must be constantly changing. Because of this, transformers may be used to change the voltage of alternating current (AC). Direct current voltages are constant voltages that do not change. The application of direct current to a transformer results in resistive heating of the coil without any voltage output. Because of this, transformers may be used to change AC voltage but will not work with DC voltage.
The current carrying capacity of wires depends on the cross sectional diameter of their conductive metal center. Power in watts is represented by current in amperes times the applied voltage. The higher the voltage the more power a wire of any given conductive metal cross section can carry. Certain household appliances require over one thousand watts of power. Such appliances include hair dryers, air conditioners, electric ovens and stoves, and large microwave ovens. In order to provide this level of power without placing excessive current demands on electrical wiring, about 115 volts is required. If more than two thousand watts of power is required higher voltages may be employed.
AC electric power may be carried across long distances using high voltage utility lines and may pass through numerous transformers before being used to do useful work. With AC power, the higher the frequency, the greater the losses in transformers and transmission lines. Because of this, 60 cycles (a relatively low frequency) was chosen. It should be noted that below about 60 cycles, the flicker may be detectable in some electric light bulbs.
As mentioned above, contact with 115 volts AC may result in disruptive levels of current across body parts. Although somewhat hazardous, 115 volts AC may represent a reasonable compromise between safety and the need to carry useful quantities of power through relatively small wires. Below about 42 volts, difficulties arise in pushing enough current through intact skin to disrupt bodily functions. This arbitrary voltage has been chosen as being relatively “safe” in that contact with voltages below 42 rarely results in serious electric shock. It should be noted that exposure to sources of electric power below 42 volts under certain circumstances may still be harmful. For example, broken skin, wet conditions, and puncture wounds by electrically energized electrical components at or below 42 volts can still cause harmful electric currents to flow within the body.
Electric arcing represents one more reason why AC power is used instead of DC power. When a voltage is applied across a coil of wire, a magnetic field is established in accordance with the right hand rule of electrically induced magnetism. This electrically induced magnetic field represents stored energy. With DC electric power this field may build up to a high level and remain at that level until the power is disconnected. On disconnection of electric power (such as turning off a switch) the magnetic field will rapidly collapse. This often results in a large reverse voltage spike that may be sufficient to strike an arc across switch contacts. This arcing tends to damage electric switches and may even result in a hazardous condition if it becomes self sustaining. AC electric power has less tendency toward arcing in switch contacts when turning off inductive loads. This reduced tendency is due at least in part to the fact that the voltage is constantly rising and falling and reversing itself.
The tendency of surface contamination to inhibit the flow of current across two electrically conductive contacting surfaces is usually overcome by the 115 volt AC household power. While being sufficient in most cases with copper connections, it is not always sufficient to overcome electrically resistive contamination that may be found on aluminum wire.
Aluminum wire was previously used in some houses as a lower cost option to more expensive copper. Aluminum is low in cost, lightweight, and is a good conductor of electricity. Unfortunately, it tends to form non-conductive surface oxides on exposure to air. Once these oxides form, a point of high resistance may develop where the wire makes contact with its connection. In some instances this oxide layer may be sufficient to impede the flow of 115 volt AC current. A large voltage drop may occur across the contact surface resulting in local heating effects. Aluminum has a relatively high coefficient of thermal expansion. Numerous expansion and contraction cycles may loosen connections. Significant currents across loose connections coupled with oxide layers of high electrical resistance may produce sufficient heat to ignite the interior surfaces of buildings resulting in fire.
While aluminum and its associated oxide forming surface layers may provide difficulties in carrying 115 volt AC power across contacting surfaces, copper and various other metals often employed in conducting electric current across electrically conductive contacting surfaces tend to be more forgiving.
Numerous metals including copper may be used to conduct 115 volt AC electric current across contacting surfaces with little difficulty. Of further interest is the ability of copper and several other metals to efficiently carry electric current between contacting surfaces at or below 42 volts.
Many electrical contactors rely on significant applied voltages to overcome barriers to the flow of electrons across both contacting metal surfaces. Many electrical contactors provide good electrical conduction when operated at a value equal to or greater than about one hundred volts. Contactors may perform well at significantly lower voltages as well. Below about 20 volts difficulties may be encountered in copper and other metal contactors conducting electricity across contaminated surfaces. As in the case with aluminum, this may present special problems associated with the unwanted formation of heat at the point of contact while carrying high currents.
The more contact surface area, points of contact, pressure, and applied voltage, the better the electrical conductivity between the two surfaces. In many instances surface contamination involves non-conductive materials. Because of this, contamination between contacting metal surfaces may reduce conductivity between them. Keeping the area clean may help to improve contact conductivity but may prove difficult. Increasing the pressure between contacting pieces of metal may help to push contamination out of the way thereby improving conductivity between them. In addition, contact surface area and or the number of actual contact points may improve. It should be noted that for electrical contact to occur between two pieces of metal loose electrons from one piece of metal need to travel over to the atoms of the other and vice versa.
A specific example of this type of electric connection is the contact area between a car battery post and battery clamp. A poorly conducting metal (lead) is in an adverse environment (sulfuric acid, vibration, changing temperatures, and galvanic effects) to carry significant amounts of current (100 to 1,000 amperes) at a relatively low voltage 12 volts DC. Battery clamps used in vehicles employ significant pressure to improve conductivity between the clamp and battery post. Unfortunately despite this fact, poor electric continuity may exist between vehicle battery posts and their associated clamps.
Despite the need for high power during starting, automotive batteries are rated at 12 volts. This voltage may represent a compromise for the need to use higher voltages to reduce the current carrying demands of electrical wiring with the need to keep battery costs down. Higher voltage batteries require more series wired cells and therefore are more expensive to produce. In addition, the more cells connected in series the greater the chances of one of the series connected cells failing. It should be noted that a 42 volt system may be used without creating an unreasonable electric shock hazard.
In addition to the main battery connection, there are numerous electrical connectors located under the hood and throughout the entire vehicle. These connectors are used to connect numerous wires to other wires, fuse boxes, sensors, circuit boards and other components requiring electric power. The majority of electrical contacting surfaces employed in vehicles are of the spring loaded sliding type designed for only a few cycles of connection and disconnection. Twelve volt automotive electrical systems employing numerous spring loaded sliding electrical connectors exposed to heat cycling, vibration, and contaminants presents certain challenges to the automotive industry. Increasing this voltage to 42 may help to improve the overall reliability of automotive electrical systems.
There are numerous inductors (coils of wire wrapped around iron cores) that are employed in automotive electrical systems. Included in this group are starter motors, alternators, electric motors for fans, windows, and windshield wipers, horns, relays, speakers, induction coils, and solenoid door locks. All of these inductors are capable of creating back voltage spikes having values several times the initial applied voltage. While being somewhat damaging to switches and relay contacts, these voltage spikes may be especially problematic to semi-conductor components found in computer chips, power regulating circuitry, and control circuitry. Of particular concern is the generation of stray unclamped voltage spikes in the electrical systems of newer vehicles. When a conductor carries an electric current, a magnetic field is established with that current in accordance of the right hand rule of electrically induced magnetism. This magnetic field builds up to a fixed level and then remains at that level as long as the conductor carries the current. This magnetic field represents stored energy. If the current is discontinued in such a conductor, the resultant magnetic field rapidly collapses. The rate of field collapse is usually much faster than the rate of build up. This rapidly collapsing magnetic field creates a voltage spike in the opposite direction that is often several times the original input voltage. When current is interrupted to a conductor having significant inductance (such as an ignition coil or alternator electromagnet) large spikes can be generated that are more than capable of permanently destroying delicate semi-conductor components such as MOSFETs (metal oxide semi-conductor field effect transistors).
In order to reduce the likelihood of damage to semi-conductor automotive components, protecting circuitry is often added to absorb voltage spikes. Voltage clamping devices such as reverse wired diodes, Zener diodes, surge protectors, RC snubbers, and the like are often employed to protect sensitive semi-conductor components from harmful voltage spikes. Many of these voltage clamping devices are used to absorb voltage spikes from common sources.
Suppression of transient voltage spikes is well known art. The following references are relevant to electrical systems used in the automotive industry and are incorporated herein by reference.                1. Betten, John. “Clamping circuit tames automotive voltage transients.” Automotive Design Line. 30 Aug. 2006. www.automotivedesignline.com.        2. Tyco Electronics. “Automotive Electronics Protection using a PolyZen Device.” 2006. Page 1. www.circuitprotection.com        3. Littlefuse, Inc. “Voltage Suppression-Solutions Tech Brief.” www.littlefuse.com        4. Dallas Semiconductors. “Integrated Voltage Limiters for Automotive Applications.” Application Note 3895. 2005. www.maxim-ic.com.        5. Berger, Ivan. “Can You Trust Your Car?.” Spectrum. www.spectrum.ieee.org/print/1419.        6. Kobe, Gerry. “The 42-Volt Revolution-Automotive Battery Increase.” Gale Group 2002.        
More detailed descriptions may be found in numerous books covering the fields of electronics and electrical engineering.
Voltage spikes generated from sources unanticipated by the designers may bypass voltage clamping devices and damage semi-conductor components. The resulting problem may be difficult to diagnose if the trouble causing voltage spikes are intermittent. Poor connections to inductive sources may produce intermittent stray voltage spikes not anticipated by designers that can damage semi-conductor components.
Today's trucks, automobiles, and SUV's rely more and more on solid state components for their efficient operation. As a result, it is important to establish good conductivity between the battery post and connector. Poor connections may result in stray voltages that can cause intermittent problems that can be difficult to troubleshoot and in some cases may damage circuit components.
With older vehicles it was standard procedure to disconnect the alternator from the battery while the engine was running as a means of testing the alternator. If the engine kept running, it was a sign that the alternator was working. If the engine stopped running it was a sign that the alternator was not working properly. Although in theory most vehicle alternators require excitation energy from the rotor coil to function, residual magnetism was often sufficient to maintain enough voltage output to keep the engine idling.
The above mentioned test procedure is generally not carried out with newer vehicles due to the possibility of stray voltage spikes damaging delicate semi-conductor circuit components.
It should be noted that an automotive battery is capable of absorbing and clamping voltage transients. An intermittent battery connection may therefore produce voltage spikes by breaking electrical connections to inductors (coils of wire on iron cores) but may also present issues with not being able to absorb spikes once they are generated from other sources. Of course much of this depends on the particular wiring configuration of the automobile.
With respect to battery connections in automobiles, the charging circuit is designed to keep the battery voltage during use at 13.8 volts. This is the value that has been generally accepted for maintaining proper charge on 12 volt lead acid batteries. Poor battery connections may interfere with feedback voltage detection and charging efficiency. This condition may result in an over charge condition or an under charge condition that can significantly reduce battery life.
One fortunate aspect of lead acid batteries is that they tend to be somewhat tolerant to having a slight overcharge or undercharge. Other rechargeable batteries commonly employed in consumer electronic devices are more sensitive. For example, Nickel metal hydride batteries do not tolerate overcharge. Overcharging these batteries may result in rapid loss of capacity and significantly shorten their useful life. Lithium ion rechargeable batteries are commonly used in portable electronic devices such as cellular telephones and lap top computers. These batteries are used because of their high energy density. One unfortunate aspect of lithium ion batteries involves overcharging. Lithium ion batteries employ lithium ions to transfer charge back and forth between a material that holds and releases them in their ionic state. Lithium ions are relatively inert and therefore pose little hazard. Lithium metal on the other hand is reactive and may explode or burst into flame on exposure to water and other substances. Once a lithium ion battery is fully charged, a slight increase in charging voltage may cause the lithium ions to gain electrons forming lithium metal. Once this happens, further charging may plate enough of this metal out on the negative electrode to puncture the separator causing an internal short circuit. Local heating from the internal short circuit may be sufficient to cause fire and expose lithium metal to ambient air. Once this occurs, the fire that results may be particularly troublesome owing to the fact that lithium metal reacts with water forming explosive hydrogen.
Because of the hazardous overcharge condition of lithium ion batteries, each individual cell within a battery pack may be provided with voltage limiting circuitry. The industry standard is to limit the charging voltage to below 4.2 volts per cell.
A bad battery connection that gives a false voltage reading or bad connections within charging circuitry for lithium ion batteries can therefore be particularly troubling owing to the hazards associated with their overcharge condition.
Below about 5 volts, small amounts of surface contamination may interfere with electric conductivity between two contacting pieces of metal. The application of significant pressure to the contacting area may help to remedy the situation.
The amount of surface contamination may be quite variable and is dependent on numerous parameters. For example, many common metals such as aluminum rapidly form thin oxide layers on exposure to air. These oxide layers tend to be rather thin at first and often self passivating. Self passivation of freshly exposed metal surfaces is the result of the newly formed oxide layer being somewhat impervious to oxygen thereby limiting the overall thickness. It should be noted that surface exposed metal atoms have less other atoms surrounding them and therefore may be in a more reactive state. Thin film self passivating oxide layers may be from a few atoms thick to a few hundred atoms thick. Such layers may present continuity issues at low voltages. In addition, it should be noted that connection and disconnection of electrically contacting surfaces under the conditions of load may further increase the formation of oxide films.
The consumer electronics industry includes numerous devices that operate at low voltages. For example, the logic circuitry used in computers and other electronic devices often operates at 5 volts. This may be due at least in part to the properties of semi-conductor junctions. Because of the low voltages employed, a significant portion of their electrical connections are soldered into place. Removable connections often employ high pressure sliding inert gold plated metal contact surfaces. Satisfactory results are obtained with these connectors because they only need to stand up to a few cycles of connection and removal.
Electrical contactors designed for numerous repeated cycles of connection and disconnection often employ two conductive surfaces that are pressed together. Spring force is often employed in switches and magnetic force is often employed in relays. Electrical contactors designed for a few repeated cycles of use often take the form of conductive pieces of metal that are designed to slide against each other employing the compression force of a spring. The spring providing this compression force is often one of the sliding contacting surfaces. An example of this type of contactor is the household outlet and pronged plug. Electrical contactors designed for single use or a very limited number of cycles often employ the squeezing together of conductive electrical contacting surfaces using screws, nuts, and bolts. The use of screws, nuts, and bolts provides an easy way of exerting very high compression forces between two electrically conductive contacting surfaces.
Low voltage electrical contacting surfaces employed in numerous switches, batteries, and applications may develop poor electric continuity over time. This may become especially problematic with repeated use. Particularly troublesome are the electrical contacting surfaces of individual cell batteries. When several cells are connected in series, numerous bad connections may result. It should be noted that single cell consumer batteries may employ a dimple on one or more contact surfaces. This dimple may be used to provide a single point of high pressure that may help to establish a good single point connection between the battery terminal and another electrically conductive contacting surface.
Despite numerous improvements there remains a need to provide electrical contacting surfaces for repeated use having good conductivity at low operating voltages.
It is an object of this invention to provide electrical contacting surfaces having good conductivity with other conductive surfaces at low voltages.
It is a further object of this invention to provide electrical contacting surfaces resistant to the effects of surface contamination.
It is a further object of this invention to provide electrical contacting surfaces suitable for use in adverse environments.
It is a further object of this invention to provide electrical contacting surfaces suitable for repeated use.
Finally it is an object of this invention to provide redundancy in electrical contact by employing conducting surfaces having numerous protrusions.